Add documenation about env variables to Mix module.
Add documenation about env variables to Mix module.

antipax/elixir,antipax/elixir,gfvcastro/elixir,kimshrier/elixir,michalmuskala/elixir,lexmag/elixir,ggcampinho/elixir,pedrosnk/elixir,beedub/elixir,beedub/elixir,ggcampinho/elixir,kimshrier/elixir,lexmag/elixir,kelvinst/elixir,joshprice/elixir,gfvcastro/elixir,pedrosnk/elixir,elixir-lang/elixir,kelvinst/elixir